The invention described herein may be manufactured and used by or for the Government of the United States of America for governmental purposes without the payment of any royalties thereon or therefor.
The present invention relates to methods and apparatuses for sealing machinery, more particularly to mechanical seal systems for rotary shafting mechanisms such as used in propulsors, pumps and compressors.
Current mechanical seal systems for the main propulsion shaft apparatus of marine vessels typically comprise either circumferential-type seals or face seals.
A known kind of circumferential seal includes packing which seals by being hydrostatically compressed from the axial clamping force of bolts acting on a stuffing box and packing gland arrangement. Other conventional methodologies of circumferential sealing include lip seals (or variations thereof) which make use of a spring to radially load an elastomer around the rotating shaft. Circumferential seals are inexpensive, but the failure mode involves leakage and shaft wear, with concomitant repairs entailing a new shaft sleeve or new shaft.
Face seals, on the other hand, have replaceable wearing elements which wear on their axial faces (i.e., perpendicular to the radial direction). These seals are easy to maintain once installed, since there is usually no or minimal leakage and there are no adjustments required. Generally, the failure mode for a face seal is leakage over time, which leads to replacement or seal face refurbishment.
A notable type of failure mechanism for propulsion shaft seals involves dirt and contamination which will wear at the sealing interface. An ideal seal behavior is one that exhibits recovery from leakage attributable to the ingestion of wear debris. The present invention has basis in the recognition that the design of the seal, including the design of the materials, plays an important role in promoting seal recovery.
Currently, the U.S. Navy almost exclusively uses face seals on its large surface ship combatants and submarines; its small combatants and commercial boats, however, most often implement circumferential seals. As compared with a circumferential seal, it is more desirable to use a face seal in smaller water craft (e.g., smaller naval combatants) because of the relative lack of attention required as well as the minimization of bilge leakage. In terms of commercial availability, there is an apparent dearth of completely split face seal designs such as would be suitable for the U.S. Navy""s smaller marine vessels. Although small unsplit face seal designs are commercially available, these do not appear to be rugged enough for the naval marine environment. Moreover, the U.S. Navy lacks knowledge and experience with the design and materials used in the commercially available xe2x80x9coff-the-shelfxe2x80x9d products. The present invention appreciates that the design and materials for a small split face seal can be improved or optimized to suit U.S. Navy and other applications.
Incorporated herein by reference are the following pertinent United States patents: Reagan U.S. Pat. No. 5,820,129 issued Nov. 13, 1998; Clark et al. U.S. Pat. No. 5,725,220 issued Mar. 10, 1998; Duffee et al. U.S. Pat. No. 5,716,054 issued Feb. 10, 1998; Clark et al. U.S. Pat. No. 5,711,532 issued Jan. 27, 1998; Bessette et al. U.S. Pat. No. 5,662,340 issued Sep. 2, 1997; Reagan U.S. Pat. No. 5,615,893 issued Apr. 1, 1997; Azibert U.S. Pat. No. 5,571,268 issued Nov. 5, 1996; Borino et al. U.S. Pat. No. 5,529,315 issued Jun. 25, 1996; Radosav et al. U.S. Pat. No. 5,490,682 issued Feb. 13, 1996; Bowers U.S. Pat. No. 5,409,241 issued Apr. 25, 1995; Carmody U.S. Pat. No. 5,354,070 issued Oct. 11, 1994; Glynn et al. U.S. Pat. No. 5,292,138 issued Mar. 8, 1994; Pecht et al. U.S. Pat. No. 5,217,233 issued Jun. 8, 1993; Radosav et al. U.S. Pat. No. 5,199,720 issued Apr. 6, 1993; McOnie U.S. Pat. No. 5,192,085 issued Mar. 9, 1993; Radosav U.S. Pat. No. 5,114,163 issued May 19, 1992; Nagai et al. U.S. Pat. No. 5,067,733 issued Nov. 26, 1991; Mullaney U.S. Pat. No. 5,020,809 issued Jun. 4, 1991; Lowe et al. U.S. Pat. No. 4,795,169 issued Jan. 3, 1989; Azibert U.S. Pat. No. 4,576,384 issued Mar. 18, 1986; Copes U.S. Pat. No. 4,410,188 issued Oct. 18, 1983; Wilkinson U.S. Pat. No. 4,239,240 issued Dec. 16, 1980; Adams U.S. Pat. No. 3,773,337 issued Nov. 20, 1973.
In view of the foregoing, it is an object of the present invention to provide a reliable face sealing system for a rotary shafting mechanism, such as a ship""s main propulsion shafting mechanism.
It is another object of the present invention to provide such a sealing system which will remain leak-free for a significant period of time, such as throughout the life cycle of a ship.
It is a further object of the present invention to provide such a sealing system which is easily maintainable.
Another object of the present invention is to provide such a sealing system which may be used in certain applications, such as in association with small naval vessels, wherein circumferential sealing systems are customarily used.
In accordance with the present invention, a mechanical face seal combination is suitable for use in connection with machinery of the type wherein an axially rotative shaft passes through an immobile housing. The inventive combination comprises a first annulus, a second annulus and support apparatus. The first annulus has a first annular face which is at least substantially planar. The second annulus has a second annular face which is at least substantially planar. The support apparatus includes biasing means. The first annulus, the second annulus and the support apparatus are adaptable to arrangement wherein: the first annulus is attached to the housing; the support apparatus is attached to the shaft; the first annular face and the second annular face coaxially abut; and, the support apparatus holds the second annulus in position with respect to the shaft whereby the biasing means contactingly presses the second annulus so as to approximately axially bias the second annular face against the first annular face.
Notable among the features of the present invention is the robust, integral construction of the rotational structure inclusive of the face (surface) designed for dynamic abutting contact with the face (surface) of the non-rotational structure. This rotational structure, viz., the second annulus, is able to directly bear, in the absence of any intermediary structure, the loading imposed by the support apparatus which holds the second annulus in place with respect to the rotational shaft. Similarly featured is the robust, integral construction of the non-rotational structure inclusive of the face (surface) designed for dynamic abutting contact with the face (surface) of the rotational structure. This rotational structure, viz., the first annulus, is able to be directly coupled with the housingxe2x80x94e.g., directly fastened to the flanged (radially annularly projecting) end portion of a ship""s stern tube.
A resultant advantage of the superior material strength and unitary construction of both the first annulus and the second annulus is the reduction of parts needed for the overall seal configuration, with the concomitant advantage of a simplified design. In particular, obviated is the functional or structural need for additional or auxiliary structurexe2x80x94whether it be of an intermediary, intervening, interpositional, dampening, buffering, holding and/or containing naturexe2x80x94used in association with: (i) the first (stationary) annulus in relation to the stationary housing; and, (ii) the second (rotational) annulus in relation to the support apparatus which holds the second annulus in place with respect to the rotational shaft.
According to many inventive embodiments, the second (rotational) annulus includes (e.g., is at least substantially made of a polymeric matrix composite characterized by sufficient structural integrity (e.g., in terms of strength, rigidity and elastic deformation) for enduring the holding by the support apparatus (under both rotating and non-rotating conditions of the shaft), including enduring the pressing by the biasing means. According to typical inventive practice, the second annulus is provided with openings (e.g., slots) and the support apparatus is provided with projections (e.g., cogs); the holding operation of the second annulus by the support apparatus includes the interlocking or mating of the openings and projections; at the same time, spring devices (or other spring-loading means) pushing off the support apparatus are exerting an axial force against the second annulus. The robust material composition of the second apparatus enables it to resist the stresses and strains associated with such modes of containment by the support apparatus.
Another feature of note according to many inventive embodiments is a complementarity of the respective material compositions of the rotational structure and the non-rotational structure, thus giving rise to the complementarity of their respective wear surfaces. According to many inventive embodiments, the first (stationary) annulus includes (e.g., at least substantially consists of) a metal matrix composite (MMC) or a ceramic matrix composite (CMC), and the second (rotational) annulus includes (e.g., at least substantially consists of) a polymer matrix composite (PMC). It is believed that the present invention""s matrix-composite-on-matrix-composite material wear combination will imbue the wear interface, on a continual basis, with xe2x80x9cself-healingxe2x80x9d (xe2x80x9cleakage-restorativexe2x80x9d) and contamination-resistant attributes.
The present invention thus features a single stationary wearing piece and a single rotational wearing piece. These inventive features are predicated on the inventive observation that a wearing face structure made of an appropriate matrix composite material will be accorded both structural and tribological properties. According to preferred practice of this invention, both the stationary wearing piece and the rotational wearing piece will be made of a matrix composite material; that is, the stationary wearing piece will be made of MMC or CMC, and the rotational wearing piece will made of PMC.
Therefore, according to this invention, both the stationary wearing piece and the rotational wearing piece will be endowed with xe2x80x9cstructuralxe2x80x9d attributes, especially in terms of load-bearing capabilities. Furthermore, the PMC material of the rotational wearing piece, in interacting with the MMC or CMC material of the stationary wearing piece, will provide xe2x80x9clubricityxe2x80x9d to the wear interface. In other words, the rotational wearing piece""s PMC material will afford a transfer of polymeric material so as to develop a xe2x80x9ctransfer filmxe2x80x9d of low shear strength, thus allowing for a low coefficient of friction against a stationary wearing piece made of a wear-resistent matrix composite material such as MMC (e.g., a bronze ceramic matrix, which is essentially a ceramic-filled bronze) or a CMC.
According to conventional seal technology, the seal faces must maintain their original flat surfaces in order for them to to actually seal. If either or both wear surfaces become xe2x80x9cout of flatxe2x80x9d or worn, then the sealing capabilities will degrade. The traditional approach is to utilize hard and brittle materials such as silicon carbide or carbon, which will tend to maintain their original flat surface condition for some period of time; however, once there is a wear spot, the seal must be removed and replaced. The conventional requirement of a high degree of flatness in a hard brittle material can be costly and time-consuming.
By contrast, according to this invention, the utilization of a combination of two kinds of matrix composite materials within a certain sealing geometry will promote leakage restoration, and therefore will not require brittle materials and precise surface conditions. According to the present invention, the flatness tolerance is relaxed due to the implementation of the selected engineered matrix composite materials. The combination of matrix composite materials can be xe2x80x9clappedxe2x80x9d in place via the natural machining action of the seal and the concomitant rubbing of the wearing surfaces. The material design is hence also contamination-resistant, since the contaminants will be subsumed in the ongoing self-healing process of the wear interface.
The present invention thus provides a lower cost seal system requiring significantly less xe2x80x9cdownxe2x80x9d time for repair or maintenance, which conventionally involves disassembly. Another benefit of using metal matrix composite and polymeric matrix composite materials is that they are both corrosion-resistant. An additional benefit of the inventive seal system, arising from its xe2x80x9cself-healing quality,xe2x80x9d is the minimization of installation space around the seal, which would traditionally be needed for maintenance and repair; the present invention thus allows for the insertion of additional machinery in spaces that would conventionally be reserved to make room for personnel.
The inventive use of wear-resistant and self-lubricating matrix composite materials will thus provide long wearing surfaces. The characteristics of the matrix composite materials can be optimized or tailored according to the particular inventive application. The selected matrix composite materials can be specially developed, custom engineered or purchased commercially (e.g., xe2x80x9coff the shelfxe2x80x9d). The matrix composite materials can be selected, according to this invention, on the basis of a general model wherein a stationary face piece made of a metal matrix material or ceramic matrix material is in rubbing contact with a rotative face piece made of a polymeric matrix material, and wherein the metal or ceramic matrix material affords wear resistance while the polymeric matrix material affords lubricity via transfer film formation. Generally, each of these kinds of matrix composite materials (viz., MMC, CMC and PMC) are castable and/or moldable materials which allow for near net shape and/or final shape, thus requiring a minimal amount of processing.
Another feature of many inventive embodiments is a completely split seal design. In other words, the entire inventive split seal assembly can be applied in place to the machinery, without any need for disassembly of the shaft line. According to many inventive embodiments, there are four main generally annular structures, viz.: (i) the stationary face structure (e.g., stationary seal face); (ii) the rotative face structure (e.g., floating seal ring); (iii) the coupling structure (e.g., drive ring), which serves to hold the rotative face structure in position with respect to the shaft); and, (iv) the clamp structure (clamp ring), which serves to clamp the coupling structure with respect to the shaft. The terms xe2x80x9cannularxe2x80x9d and xe2x80x9cring-shaped,xe2x80x9d as used herein, refer to a shape which generally, substantially, essentially or approximately describes that of a circular ring. All four ring-shaped structure basically describe an overall annular shape when -viewed elevationally in the axial direction; further, each ring-shaped structure has its own cross-sectional geometry. All of these ring-shaped structures lend themselves to having a xe2x80x9csplitxe2x80x9d construction wherein two semi-annular halves can be united (into one annular piece) and disunited (into two semi-annular pieces).
According to this invention, there is no need for any complex locking mechanism (e.g., such as would have to account for seawater), or for any difficult machining process such as would be applied to the shaft sleeve or shaft. In this regard, the present invention provides for uncomplicated engagement of the stationary face ring with the housing, of the rotational face ring with its support ring, and of the face ring""s support ring with its clamp ringxe2x80x94all requiring no real structural modification (other than the machining of holes). This is especially beneficial for applications wherein the shaft, shaft sleeve and or other parts are made of composite materials.
Inventive practice can also admit of configurational adjustment of the inventive seal system under damaged conditions. Parts of an inventive face seal system can be removed and rearranged, with packing introduced, thereby basically xe2x80x9csalvagingxe2x80x9d the original component; for continous use, through minimal effort.